Caecilians: Gymnophiona

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CAECILIANS: Gymnophiona


Caecilians (sih-SILL-yuhns) are long, legless amphibians with a dual-action jaw. Some have tails, but most do not. Amphibians (am-FIB-ee-uhns) are vertebrates (VER-teh-brehts), or animals with a backbone, that have moist, smooth skin; are cold-blooded, meaning their body temperature is the same as the temperature of their surroundings; and, in most instances, have a two-stage life cycle. Some species of caecilians are land dwellers, and some are water dwellers.

The most striking feature of caecilians is a series of rings that make these amphibians look like earthworms. The rings run the length of the body starting just behind the head. The rings are inside the body and attached to the vertebrae (VER-teh-bree), or the bones that make up the spinal column. Some caecilians have one ring per vertebra (VER-teh-bruh, the singular of vertebrae); some have two rings, especially toward the rear of the animal; and some have three. The skin is folded over the rings, making grooves between the rings. In some species the grooves go all the way around the body, and in some they only go part of the way around. The second and third sets of rings make shallower grooves than the main set. The species with three rings per vertebra have a short tail, and the tail also has rings. Some species of caecilians have scales just under their skin.

Between the nostrils and the eyes on each side of the head, caecilians have a tentacle that is pumped out and in as needed. The tentacles are sense organs that respond to chemicals and touch. Inside each tentacle is a fluid-filled channel that runs from the tip of the tentacle to a chamber that opens into the part of the brain that controls the sense of smell. There is a hole in the skull through which the tentacle pumps out and in. This hole can be anywhere between the eyes and the nostrils. Scientists use the location of the hole to tell different species of caecilians apart.

The eyes of caecilians are small and covered by skin or, in some species, bone. The eyes of many species have lost some or all of their muscles, and some have lost the lens and have only a small retina and optic nerve. Even with the lack of eye parts, most caecilians can distinguish light and dark.

Caecilians have a somewhat flat head with nostrils at the tip of the snout and a large mouth. In some caecilian species the lower jaw is the same length as the upper jaw, and the mouth opens at the front of the animal's head. In other species, the lower jaw is shorter than the upper jaw, a feature that improves burrowing ability. The mouths of these species open on the bottom of the animal's head. Caecilians have two rows of teeth on the upper jaw and one or two rows on the lower jaw. The tops of the teeth have different shapes among species, but all teeth are hinged and usually curve backward, apparently to prevent the loss of prey the animals have caught.


All vertebrates except caecilians have a single pair of muscles used for closing their jaws. These muscles pull up on the lower jaws. Caecilians have this mechanism of jaw closure, but they also have a second mechanism in which another pair of muscles assists in closing the jaws. The second muscle pulls down on part of the lower jaw, causing the forward, toothed portion of the lower jaw to close by rotating upward. The action is much like that of a seesaw.

The skin of caecilians has glands that release a substance that is poisonous to many predators. Most caecilians are dark gray to grayish brown to deep purple, often with a lighter head and belly. Some caecilians, however, are brightly colored. For example, a species on the African island of São Tomé is bright yellow. A South American species is deep bluish purple with bright white grooves between its rings. Some species are dark gray to grayish brown to brownish black with bright yellow side stripes. Most adult caecilians are 12 to 24 inches (30 to 60 centimeters) long, but some species are much smaller or larger. The smallest adult caecilians are about 4.5 inches (11 centimeters) long. The largest are more than 63 inches (160 centimeters) long.


In the Western Hemisphere, caecilians live in an area that extends from central Mexico through Peru, Bolivia, Paraguay, and northern Argentina. The water-dwelling or partially water-dwelling species live mainly in the regions of the Orinoco and Amazon rivers, but some live in an area that extends from Colombia to northern Argentina and Uruguay. In the Eastern Hemisphere caecilians live in eastern and western Africa except the Sahara, in the Seychelles, India, Sri Lanka, Southern China, Cambodia, Laos, Vietnam, and much of Malaysia to the southern Philippines. Caecilians do not live in Madagascar, much of central Africa, or Australia.


Some caecilians live in moist soil that is rich in decayed plant matter. They also live in leaf litter and sometimes even in the lower parts of plants. Other caecilians live in water all or most of the time.


Caecilians are meat eaters. Land-dwelling species prey on animals that they can reach on the ground, including earthworms; termites; insects, such as crickets, that have shed their outer layer; and many other invertebrates (in-VER-teh-brehts), or animals without backbones. Large caecilians have been known to eat lizards and baby rodents. The water-dwelling species nose about at the water's bottom to find food, or they scrape food from logs and rocks. They eat water insects, shrimp, and small fish.


Caecilians look like giant earthworms with a big mouth, but unlike earthworms, caecilians are vertebrates, which means they have backbones. In addition, caecilians don't just look like earthworms; they eat them!


Caecilians are excellent burrowers in the ground or in leaves. They pump their tentacles in and out while they are moving in order to investigate their surroundings. Caecilians sometimes twist their bodies rapidly when subduing prey that they have grasped with their mouths. Scientists do not know how water-dwelling and partially water-dwelling caecilians behave because these animals live in dark, cloudy water and are hard to observe.

Some caecilians lay eggs that hatch into free-living larvae that have small gills and tail fins. Larvae (LAR-vee) are animals in an early stage that change body form in a process called metamorphosis (MEH-tuh-MORE-feh-sis) before becoming adults. Gills are organs for obtaining oxygen from water. Scientists believe the eggs are laid in burrows or under grass or leaf litter on land. The female guards the eggs until the newly hatched larvae wriggle into nearby streams, where they live until metamorphosis. After metamorphosis the caecilians again become land dwellers. Other caecilians go through metamorphosis while inside the eggs, so they hatch with the body form of adults.

While inside the female, some species of caecilians have fetal (FEE-tehl) teeth that are different from adult teeth. The developing young use the teeth to chew a nutrient liquid made by the inner lining of the egg tubes inside the mother and to stimulate production of this liquid. The fetal teeth are shed at or near birth.


Caecilians eat insects that are harmful to people. The burrowing movements of land-dwelling caecilians turn soil and thus keep it in good condition.


The World Conservation Union (IUCN) lists one species of caecilians as Endangered and three species as Vulnerable. Endangered means facing very high risk of extinction in the wild. Vulnerable means facing high risk of extinction in the wild.



Duellman, William E., and Linda Trueb. Biology of Amphibians. Baltimore, MD: Johns Hopkins University Press, 1994.

Lamar, William W. The World's Most Spectacular Reptiles and Amphibians. Tampa, FL: World, 1997.

Lawlor, Elizabeth P. Discover Nature in Water and Wetlands. Mechanicsburg, PA: Stackpole, 2000.

Llamas Ruiz, Andres. Reptiles and Amphibians: Birth and Growth. New York: Sterling, 1996.

Petranka, J. W. Salamanders of the United States and Canada. Washington, DC: Smithsonian Institution Press, 1998.

Web sites:

"Caecilian." Animal Bytes. (accessed on April 11, 2005).

Hawes, Alex. "On Waterdogs, Mudpuppies, and the Occasional Hellbender." Zoogoer. (accessed on April 11, 2005).

Summers, Adam. "Squeeze Play." Natural History. (accessed on April 11, 2005).